Recent research findings have shown that fine, sub-micron particles suspended in the air cause a much greater negative health effect than had previously been suspected. This finding will very likely result in more stringent ambient air quality standards than the current PM-10 (particles less than 10 micrometers). The standard will likely be reduced to PM-2.5 or even PM-1. The state implementation plans that will develop in response to the more stringent Federal ambient air quality standards will require that many of the existing particulate control devices on coal-fired utility boilers and other industrial sources be upgraded to reduce their emission of the sub-micron fine particles.
Electrostatic precipitators are used as the particulate control device on about 90% of the coal-fired electric utility boilers and many major industrial plants. A large number of these electrostatic precipitators will require upgrading to meet the expected more stringent limits on emission of sub-micron fine particles.
U.S. Pat. No. 5,024,681 entitled "Compact Hybrid Particulate Collector" discloses a combination of an electrostatic precipitator and, downstream of the electrostatic precipitator and in series therewith, a fabric filter baghouse. U.S. Pat. No. 5,024,681 asserts an improvement in the performance of the "barrier filter" due to the residual charge on the particles exiting the electrostatic precipitator. However, particles exiting the electrostatic precipitator do not contain, because of various losses, the maximum electric charge. After traveling through the ductwork connecting the electrostatic precipitator to the downstream filtration unit, a good portion of the electric charge is lost from the particles. Further, this device incurs a significant gas-side pressure drop, an energy penalty typical of fabric filters and may require additional fans and space not readily available.
A "Compact Hybrid Particulate Collector" (COHPAC) is disclosed in U.S. Pat. No. 5,158,580, issued Oct. 27, 1992. In this device a conventional fabric filter replaces the last section of an electrostatic precipitator. Recent tests have indicated that this device introduces a higher than expected and desired pressure drop. The patent teaches that some of the particles that enter the array of bags have an electrical charge. As is well known to workers in the field of electrostatics, as many as half of the particles exiting from an electrostatic precipitator have been previously collected on the grounded collector plates from whence they have been reentrained back into the gas stream upon the cleaning or rapping of the plates. Upon being collected onto the grounded plates, their electrical charge is drained off, causing many of the particles exiting the electrostatic precipitator to be uncharged. The remaining particles have a relatively low level charge.
Our previous invention "Enhancement of Electrostatic Precipitation with Electrostatically Augmented fabric Filtration," U.S. Pat. No. 5,217,511, issued Jun. 8, 1993, taught that it is possible to have an electrostatic precipitator with conventional collector and electrode sections followed by an electrostatically enhanced fabric filtration section. That patent further teaches that the electrostatically enhanced filtration section increases the overall particulate collection beyond what would be possible by an electrostatic precipitator alone. The disclosed filtration unit has corona discharge electrodes interspersed among the filter bags to charge the incoming particulate matter. The electric field established between the corona discharge electrodes and the filter bags causes the charged particles to collect upon the filter bags non-uniformly.
The heaviest deposit of particles in the apparatus of our previous patent was upon the first bags in the direction of gas flow, and the lightest deposit upon those bags last in the direction of gas flow. U.S. Pat. No. 5,217,511 further teaches that the non-uniform deposit caused the overall pressure drop to be less than for a filtration section without electrostatic enhancement and, consequently, with a uniform deposit. One other aspect taught by U.S. Pat. No. 5,217,511 is the use of perforated gas diffusion plates placed before the electrostatic precipitator section to maintain good gas velocity distribution for the gas exiting the electrostatic precipitator sections.
However, in operation of the filtration apparatus disclosed in U.S. Pat. No. 5,217,511 it was found that if the electrical resistivity of both the particulate matter and the filter bags is excessively high the result is a condition well known in the art of electrostatic precipitation called back ionization or back corona. This is caused when the product of the current density (Amperes/cm.sup.2) and the resistivity (ohm-cm) achieves an electric field of 5000-10,000 volts/cm. When this occurs the gas in the interstitial spaces of the particulate mater and the filter media ionizes and injects ions of opposite polarity into the gas space that disrupts the charging process and non-uniform collection mechanism. It was further found that when the corona discharge electrodes were energized with sufficient voltage to cause corona, that the likelihood of sparking between the corona discharge electrodes and the filter bags was increased. Excessive sparking tends to cause the formation of holes (=leaks) in the fabric of the filter bags. To overcome the likelihood of sparking it is necessary to maintain good alignment of the corona discharge electrodes and the filter bags, and good voltage control to maintain operation below the sparking limit, which measures both add expense to the system. Finally the diffusion plates add to the gas flow pressure drop, and consequently to power consumption and its cost, due to the greater force needed to cause the gas to flow through the system.